The present invention relates to methods for improving the cold start capability of an electrochemical fuel cell. More particularly, the present invention relates to temperature dependent methods for improving the cold start capability of fuel cell electric power generation systems that include a fuel cell stack.
Electrochemical fuel cells convert fuel and oxidant to electricity and reaction product. Solid polymer electrochemical fuel cells generally employ a membrane electrode assembly (xe2x80x9cMEAxe2x80x9d) which comprises an ion exchange membrane or solid polymer electrolyte disposed between two electrodes typically comprising a layer of porous, electrically conductive sheet material, such as carbon fiber paper or carbon cloth. The MEA contains a layer of catalyst, typically in the form of finely comminuted platinum, at each membrane/electrode interface to induce the desired electrochemical reaction. In operation the electrodes are electrically coupled to provide a circuit for conducting electrons between the electrodes through an external circuit.
At the anode, the fuel stream moves through the porous anode substrate and is oxidized at the anode electrocatalyst layer. At the cathode, the oxidant stream moves through the porous cathode substrate and is reduced at the cathode electrocatalyst layer to form a reaction product.
In fuel cells employing hydrogen as the fuel and oxygen-containing air (or substantially pure oxygen) as the oxidant, the catalyzed reaction at the anode produces hydrogen cations (protons) from the fuel supply. The ion exchange membrane facilitates the migration of protons from the anode to the cathode. In addition to conducting protons, the membrane isolates the hydrogen-containing fuel stream from the oxygen-containing oxidant stream. At the cathode electrocatalyst layer, oxygen reacts with the protons that have crossed the membrane to form water as the reaction product. The anode and cathode reactions in hydrogen/oxygen fuel cells are shown in the following equations:
Anode reaction: H2xe2x86x922H++2exe2x88x92
Cathode reaction: xc2xdO2+2H+2exe2x88x92xe2x86x92H2O
In typical fuel cells, the MEA is disposed between two electrically conductive fluid flow field plates or separator plates. Fluid flow field plates have at least one flow passage formed in at least one of the major planar surfaces thereof. The flow passages direct the fuel and oxidant to the respective electrodes, namely, the anode on the fuel side and the cathode on the oxidant side. The fluid flow field plates act as current collectors, provide support for the electrodes, provide access channels for the fuel and oxidant to the respective anode and cathode surfaces, and provide channels for the removal of reaction products, such as water, formed during operation of the cell. Separator plates typically do not have flow passages formed in the surfaces thereof, but are used in combination with an adjacent layer of material which provides access passages for the fuel and oxidant to the respective anode and cathode electrocatalyst, and provides passages for the removal of reaction products. The preferred operating temperature range for solid polymer fuel cells is typically 50xc2x0 C. to 120xc2x0 C., most typically about 75xc2x0 C.-85xc2x0 C.
Two or more fuel cells can be electrically connected together in series to increase the overall power output of the assembly. In series arrangements, one side of a given fluid flow field or separator plate can serve as an anode plate for one cell and the other side of the fluid flow field or separator plate can serve as the cathode plate for the adjacent cell. Such a multiple fuel cell arrangement is referred to as a fuel cell stack, and is usually held together in its assembled state by tie rods and end plates. The stack typically includes inlet ports and manifolds for directing the fluid fuel stream (such as substantially pure hydrogen, methanol reformate or natural gas reformate, or a methanol-containing stream in a direct methanol fuel cell) and the fluid oxidant stream (such as substantially pure oxygen, oxygen-containing air or oxygen in a carrier gas such as nitrogen) to the individual fuel cell reactant flow passages. The stack also commonly includes an inlet port and manifold for directing a coolant fluid stream, typically water, to interior passages within the stack to absorb heat generated by the fuel cell during operation. The stack also generally includes exhaust manifolds and outlet ports for expelling the depleted reactant streams and the reaction products such as water, as well as an exhaust manifold and outlet port for the coolant stream exiting the stack. In a power generation system various fuel, oxidant and coolant conduits carry these fluid streams to and from the fuel cell stack.
When an electrical load (comprising one or more load elements) is placed in an electrical circuit connecting the electrodes, the fuel and oxidant are consumed in direct proportion to the electrical current drawn by the load, which will vary with the ohmic resistance of the load.
Solid polymer fuel cells generally employ perfluorosulfonic ion exchange membranes, such as those sold by DuPont under its NAFION trade designation and by Dow under the trade designation XUS 13204.10. When employing such membranes, the fuel and oxidant reactant streams are typically humidified before they are introduced to solid polymer fuel cells so as to facilitate proton transport through the ion exchange membrane and to avoid drying (and damaging) the membrane separating the anode and cathode of each cell.
Each reactant stream exiting the fuel cell stack generally contains water. The outlet fuel stream from the anodes generally contains the water added to humidify the incoming fuel stream plus any product water drawn across the membrane from the cathode. The outlet oxidant stream from the cathodes generally contains the water added to humidify the incoming oxidant stream plus product water formed at the cathode.
In some fuel cell applications, such as, for example, motive applications, it may be necessary or desirable to commence operation of a solid polymer electrolyte fuel cell stack when the stack core temperature is below the freezing temperature of water. As used herein, the freezing temperature of water means the freezing temperature of free water, that is, 0xc2x0 C. at 1 atmosphere. It may also be necessary or desirable when ceasing operation of the solid polymer fuel cell stack to improve the cold start capability and freeze tolerance of the stack by reducing the amount of water remaining within the fuel, oxidant and coolant passages of the stack. Upon freezing, water remaining within stack passages will expand and potentially damage structures within the stack such as, for example, the membrane/electrocatalyst interface, the reactant passageways, conduits and seals, as well as the porous electrode substrate material.
If there is an expectation that a solid polymer fuel cell stack will be subjected to cold temperatures, especially temperatures below the freezing temperature of water, one or more special start-up and shutdown techniques may be used. These techniques may improve the cold start capability and freeze tolerance of the stack, and improve the subsequent fuel cell performance. A measure of electrochemical fuel cell performance is the voltage output from the cell for a given current density. Higher performance is associated with a higher voltage output for a given current density or higher current density for a given voltage output.
A first method of ceasing operation of an electric power generation system improves the cold start capability and freeze tolerance of fuel cell stacks by reducing the amount of water remaining within the passages of the stack. The stack comprises a fuel cell stack connectable to an external electrical circuit for supplying electric current to the external circuit. The stack comprises at least one fuel cell comprising a membrane electrode assembly comprising an anode, a cathode, and an ion exchange membrane interposed between the anode and the cathode. The at least one fuel cell further comprises a fuel stream passage for directing a fuel stream to the anode and an oxidant stream passage for directing an oxidant stream to the cathode. Each of the streams is flowable to the fuel cell stack. The method comprises the sequential steps of:
(a) interrupting the supply of electric current from the fuel cell stack to the external circuit;
(b) purging water from at least one of the passages.
Although both the oxidant and fuel stream passages may be purged, it has been found that purging of only the oxidant stream passages generally gives satisfactory results. Thus, in a preferred embodiment of the method, the at least one of the passages is the oxidant stream passage. Step (a) preferably further comprises decreasing the flow rate of at least one of the incoming reactant streams.
The purge in step (b) may be performed at a temperature within the normal stack operating temperature range, however it has been found to be advantageous to significantly reduce the temperature of the fuel cell prior to purging one or both of the reactant stream passages. Thus in a preferred embodiment of a method of ceasing operation of an electric power generation system, the method comprises the sequential steps of:
(a) interrupting the supply of electric current from the fuel cell stack to the external circuit;
(b) reducing the temperature of the fuel cell stack to below its normal operating temperature;
(c) purging water from at least one of the passages.
Preferably in step (b) the temperature is reduced to a predetermined temperature threshold below the normal stack operating temperature before the purge is initiated. The threshold is greater than the freezing temperature of water, and preferably at least about 20xc2x0 C. below the normal stack operating temperature. It is more preferably in the range of about 15xc2x0 C. to 30xc2x0 C., and still more preferably less than about 10xc2x0 C.
The nominal operating temperature of the stack may be measured directly (for example, by locating a temperature sensor at one or more locations within the stack) or indirectly, for example, by monitoring the temperature of one or more of the fluid streams exiting the stack. In practice, measurements such as these may be used to provide or infer a representative or approximate value for the stack operating temperature.
In the above embodiments of a method, preferably the water is purged from the passages by flowing a fluid stream therethrough. The fluid stream may be, for example, an inert liquid or gas (such as nitrogen) or one of the reactant streams. The water carrying capacity of a gas increases with decreasing gas pressure, so if a gas is used to purge the passage preferably the pressure of the gas is not greater than about 30 psig (207 kPa gauge), and is preferably less than about 5 psig (34 kPa gauge). If both the fuel and reactant gases are to be purged simultaneously, preferably the pressure differential across the membrane during the purge is maintained at less than about 10 psi (69 kPa), and preferably less than about 5 psi (35 kPa).
Optionally, the foregoing system further comprises an incoming fuel stream with a fuel stream humidifier for producing a humidified fuel stream from the incoming fuel stream, and/or an incoming oxidant stream with an oxidant stream humidifier for producing a humidified oxidant stream from the incoming oxidant stream. If the fluid stream used to purge the at least one passage is one of the reactant streams, the respective reactant stream is flowed to purge the passage such that the respective humidifier is bypassed.
The fuel cell stack may further comprise a passage for flowing a coolant stream. If the coolant is water or another coolant that may freeze at the anticipated stack storage temperature, a preferred method includes an additional step comprising purging the coolant from the coolant stream passage. The coolant is preferably purged from the coolant stream passage by directing a fluid stream through the coolant stream passage. The fluid stream can be, for example, the incoming oxidant stream or an inert stream such as nitrogen.
The foregoing purge techniques are effective in situations in which the temperature of at least a portion of the membrane electrode assembly is subsequently to be reduced to below the freezing temperature of water.
A first method of commencing operation of an electric power generation system expedites the warming of the fuel cell stack to within its desired operating temperature range. The system comprises a fuel cell stack connectable to an external electrical circuit for supplying electric current to the external circuit. The stack comprises at least one fuel cell, the at least one fuel cell comprising a membrane electrode assembly comprising an anode, a cathode, and an ion exchange membrane interposed between the anode and the cathode. The system further comprises a fuel stream and an oxidant stream, each of the streams being flowable to the fuel cell stack. The system further comprises a coolant fluid stream flowable in thermal contact with the fuel cell stack. The method comprises:
supplying electric current from the fuel cell stack to the external circuit such that the temperature of the at least one fuel cell increases; and
flowing the coolant fluid stream in thermal contact with the fuel cell stack only after the operating temperature of the stack exceeds a predetermined temperature threshold.
The nominal operating temperature of the stack may be measured directly (for example, by locating a temperature sensor at one or more locations within the stack) or indirectly, for example, by monitoring the temperature of one or more of the fluid streams exiting the stack. In practice, measurements such as these may be used to provide or infer a representative or approximate value for the stack operating temperature.
The temperature threshold at which flow of coolant is commenced is preferably greater than about 0xc2x0 C., but may be below the typical desired operating temperature range of the fuel cell stack. For example, the threshold could be in the range of about 30xc2x0 C. to 50xc2x0 C., or the threshold may be within the desired operating temperature range, which for a solid polymer fuel cell is typically about 75xc2x0 C. to 85xc2x0 C. Once the desired operating temperature range is reached, conventional temperature regulation techniques may be used thereafter to keep the fuel cell stack operating within the desired temperature range.
This method is especially useful for commencing operation when at least a portion of the membrane electrode assembly has a temperature below the freezing temperature of water.
In an improvement upon the foregoing method, the predetermined temperature threshold at which flow of coolant is commenced is higher than the normal desired operating temperature of the stack. For example, it is preferably at least about 10xc2x0 C. above the normal desired operating temperature of the stack. For a typical solid polymer fuel cell the preferred operating temperature range may be, for example, about 75xc2x0 C. to 85xc2x0 C. In this embodiment of the method, flow of coolant could be delayed until the operating temperature reaches a value in the range of about 95xc2x0 C. to 105xc2x0 C.
This in situ xe2x80x9cheat treatmentxe2x80x9d of a membrane electrode assembly after a cold start has been shown, in certain situations, to improve subsequent fuel cell performance of a fuel cell, relative to commencing operation without operating the cell above its normal operating temperature range. Again, this improved method is especially useful for commencing operation when at least a portion of the membrane electrode assembly has temperature below the freezing temperature of water, and particularly operation on air (rather than a substantially pure oxidant). Operationally, the in situ heat treatment method can be accomplished in a number of other ways, besides delaying flow of a coolant.
Thus, a second method of commencing operation of an electric power generation system includes a period in which the stack is operated above its normal operating temperature. The system comprises a fuel cell stack connectable to an external electrical circuit for supplying electric current to the external circuit. The stack comprises at least one fuel cell, the at least one fuel cell comprising a membrane electrode assembly comprising an anode, a cathode, and an ion exchange membrane interposed between the anode and the cathode. The system further comprises a fuel stream and an oxidant stream, each of the streams being flowable to the fuel cell stack. The system optionally further comprises a coolant fluid stream flowable in thermal contact with the fuel cell stack. The method comprises:
supplying electric current from the fuel cell stack to the external circuit such that the temperature of the at least one fuel cell increases to a temperature above the normal operating temperature range of the stack; and
reducing the operating temperature of the stack to with the normal operating temperature range.
The temperature above the normal operating temperature is typically predetermined. In preferred embodiments of the method, the fuel cell stack is temporarily operated at least about 10xc2x0 C. above its normal desired operating temperature. For a typical solid polymer fuel cell the preferred operating temperature range may be, for example, about 75xc2x0 C. to 85xc2x0 C., so that stack may preferably be operated at a value in the range of about 95xc2x0 C. to 105xc2x0 C. for some period before operation in the range about 75xc2x0 C. to 85xc2x0 C. is resumed. The duration for which the stack is operated at the higher temperature may be variable, or it may be for a predetermined duration. For example, the stack may be operated at the higher temperature for about 1-2 minutes or for a few seconds. However, either or both of the temperature and duration of the higher temperature operation phase may be adjusted in response to some monitored operational parameter of the fuel cell system. For example, the preferred temperature and/or duration may depend on the temperature of the surrounding environment, the moisture conditions within the stack, a parameter indicative of reactant quality or purity, for how long the stack was stored at a low temperature, or an electrical parameter indicative of fuel cell performance.
As an alternative to the above heat treatment method, there may be some advantages to heating a fuel cell from below the freezing point of water to above its normal operating temperature range prior to commencing operation thereof. For example, an externally powered heater could be used to heat the stack or to heat a coolant circulated through the stack, or a hot fluid stream from elsewhere in the system could be used. Preferably as gas stream is circulated through one or both of the reactant stream passages during the heat treatment.
The methods described above for ceasing and commencing operation of a fuel cell may be used together or separately. In any of the above methods the exothermic operation of the stack tends to raise the operating temperature of the stack. However, other means may be used, in addition, to accelerate or facilitate the increase in temperature to within or beyond the desired stack operating temperature range.